1. Field of the Invention
The present invention relates to an intraocular lens insertion tool used to insert an intraocular lens into the eye.
2. Description of the Related Art
One method employed in the past in the field of cataract surgery involves extracting the intracapsular crystalline lens through a surgical incision made in ocular tissue such as the cornea (sclera) or anterior capsule section of the crystalline lens, and once the crystalline lens has been removed, inserting an intraocular lens serving as a replacement for the crystalline lens back through the incision and positioning it within the capsule.
Particularly in recent years, methods that employ an intraocular lens insertion tool like that disclosed in Japanese Patent No. 3412103 or Japanese Patent No. 3420724 have come into widespread use. Typically, the intraocular lens will be inserted into the eye by first inserting the distal orifice of an insertion tube provided at the distal end section of the body of the tool through a surgical incision, then pushing the intraocular lens (which has been maintained in a state of compact deformation inside the body of the tool) out through the distal orifice of the insertion tube. By employing such an insertion tool, the intraocular lens can be inserted into the eye without expanding the surgical incision that was made for the purpose of extracting the crystalline lens, thereby reducing the labor entailed in the surgical operation, as well as reducing the risk of post-surgical astigmatism or infection.
One requirement of an intraocular lens insertion tool of this type is excellent ease of control so that the surgeon will be able to consistently perform delicate operations with high precision during a surgical operation. This ease of control refers to the ability of the surgeon to carry out the lens ocular insertion procedure in the intended fashion, and requires that the intraocular lens be pushed and inserted into the eye at the intended location, in the intended condition and at the intended speed.
To meet this need, there has been proposed, for example in Japanese examined Patent Publication No. JP-B-07-079826, an insertion tool furnished in the interior of the insertion tool with a first stage drive mechanism adapted to push and advance the intraocular lens at high speed to a location slightly prior to that at which the lens is pushed out from the insertion tool, and a second stage drive mechanism adapted to subsequently push and advance the intraocular lens at low speed. According to this insertion tool, the overall procedure time of the plunging step can be reduced by advancing the intraocular lens at high speed by the first stage drive mechanism, while at the same time ensuring careful insertion into the eye by advancing the intraocular lens at low speed by the second stage drive mechanism when the intraocular lens is pushed out from the insertion tool.
However, an insertion tool like that disclosed in JP-B-07-079826 is merely a design adapted to increase the plunging speed of the plunger member in the approach zone, as such, it affords no particular advantages in relation to the lens insertion procedure per se. This insertion tool is not capable of permitting the surgeon to carry out the lens ocular insertion procedure in the intended fashion. Specifically, it is necessary for the surgeon to insert the lens into the eye at his or her own chosen speed, making it undesirable for a sudden change in perceived speed of pushing of the lens to occur at a moment just prior to pushing out the lens. That is, in order enable the surgeon to carry out the lens ocular insertion procedure in the intended fashion, it will be desirable for the surgeon, working with a constant level of pressing force, to be able to move the plunger member at constant speed up to the end and to insert the intraocular lens into the eye at constant speed.
If insertion tools of conventional design are examined from this standpoint, a satisfactory insertion tool has yet to be encountered.
As mentioned previously, the intraocular lens is made of elastic material so as to allow the surgical incision to be smaller during insertion, and will be inserted into the eye while held in a compacted (e.g. a folded or rolled) state within the insertion tube section of the insertion tool. For this reason, when the intraocular lens is pushed out from the distal end orifice of the insertion tube section within the eye, the intraocular lens will exhibit recovery so as to expand based on its inherent elasticity in the course of being pushed out partially from the distal end orifice. As result of this recovery force acting on the distal end orifice, the reaction force produced thereby will act in a direction causing the lens to be ejected from the insertion tube section.
For this reason, despite the fact that the surgeon operates the plunger member with a constant level of force, as result of the ejecting action produced by the elasticity of the intraocular lens per se, resistance against pushing of the plunger member will decrease gradually, owing to additive action of elastic recovery force and a reduction in sliding resistance of the intraocular lens with respect to the insertion tube. As a result, the intraocular lens may become pushed out at high speed before the surgeon realizes it, with a risk that the lens will ultimately pop out into the eye. This will make it difficult for the surgeon to accomplish insertion in the intended fashion, and poses a significant problem in terms of the surgeon being able to accomplish the insertion procedure in the intended fashion.
Even where the insertion tool, like that disclosed in JP-B-07-079826, has been designed to shift the plunging speed to low speed at a stage prior to traveling the prescribed distance, insofar as a plunging operation at constant force at a prescribed stroke is employed, the problem of a tendency for the intraocular lens to experience decreasing resistance at an accelerating rate as it is pushed out from the distal end orifice so that the intraocular lens flies out before the surgeon realizes it, will remain unchanged.
To address this problem, as with the insertion tool disclosed in U.S. Pat. No. 4,765,329 for example, the idea has been entertained to incorporate a coil spring and to gradually vary the operating force of the plunger member. However, with the insertion tool disclosed in U.S. Pat. No. 4,765,329, there will be an unavoidable increase in the number of parts and the number of assembly steps; and more than anything else the need for space for installation of the coil spring will make the insertion tool larger in scale and increase its weight, resulting in problems such as diminished control. Practical utility will be poor as a result.